SHORT-RANGE ELECTRON-ION INTERACTION EFFECTS IN CHARGING THE ELECTROACTIVE POLYMER-FILMS

This paper has analyzed the effects of the polymer/solution interfacial potential as a function of electrode polarization as well as of various short-range interactions between the charged components of the polymer phase, electronic and ionic species. Charging/discharging processes depend crucially on whether the value of interaction parameter is greater (repulsion or weak attraction between the species) or less (sufficiently strong attraction) than its critical value. In the former case it can be realized as a ''continuous'' transition between the insulative and conductive states of the film at sweeping polarization, the difference between the him charging and discharging (''hysteresis'') being solely due to conventional relaxation processes (diffusion, slow interfacial transfer etc). Anodic and cathodic current peaks are generally non-symmetrical, due to the film/solution potential variation. In the latter case, two quasi-equilibrium states, low- and high-density ones, coexist within a polarization range separated by a free-energy barrier so that the charging/discharging process has features of a phase transition. It may lead to a considerable hysteresis during the cycling process, even if it is realized in a quasi-equilibrium way. Variable behaviour has been found for the partial interfacial potentials versus electrode polarization, phi(m/p)(E) and phi(p/s)(E), depending on individual charges of electronic and ionic species, on the value of the global interaction parameter as well as on the ratio of different contributions to the free energy, due to electron-electron, electron-ion and ion-ion short-range interactions. These curves may be of an S- or Z-shape, possess extrema or even a loop with the self-crossing point. Highly narrow peaks are characteristic of these systems with a strong attraction between the species. There is again an asymmetry between the anodic and cathodic peaks as well as with respect to peak potentials, due to the film/solution potential variation. The ratio of anodic and cathodic peak widths depends, on the particular type of the rate-determining process, eg slow electron or ion interfacial transfer or ''droplets'' formation.